Thermophotovoltaic (TPV) energy conversion is a process of converting heat into electricity via photons. A basic TPV system includes a thermal emitter and a receiver that includes the photovoltaic (PV) cell (also commonly referred to as a solar cell). The emitter is kept at a certain (high) temperature by a heat source, and radiates photons to the PV cell. A PV cell is a semiconductor, characterized by a band gap Eg. When absorbing the emitter-radiated photons of energy larger than the band gap, electrons occupying the valence bands are excited to the conduction band, and become sources of an electric current.
The factors governing the TPV efficiency includes the number of photons emitted by the thermal emitter and the (energy) spectrum of photons absorbed by the PV cell. For a given emitter radiation spectrum, photons of low and high energies cannot be fully converted into the electric current: the low-energy photons, on the one hand, are wasted as they cannot generate conducting electrons; the high-energy photons, on the other hand, generates hot electrons which decays quickly to the conduction band bottom (via emitting phonons), and the net voltage is at most Vg=Eg/|e|. The ideal emitter spectrum is, therefore, a narrow-width spectrum peaked slightly above the band gap, with peak intensity as strong as possible.
To improve the TPV efficiency, the thermal emitter has to emit photons with energy higher than energy of the bandgap of the photovoltaic cell. Previous works, including U.S. Ser. No. 15/347,961, concentrate on designs of the thermal emitters that converts heat into radiation having surface resonant frequency tailored to the TPV systems. For example, for the near-field based TPV system, the emitter supports a few (at least one) resonant modes which are spatially localized, and the separation between the emitter and the PV cell is much shorter than the wavelength defined by the resonant modes. If fulfilled, the absorbed radiation spectrum is peaked at the resonant energies, with the peak intensity a few orders higher than the blackbody limit.
Generally, increasing the number of photons (whose energy is larger than Eg) absorbed by PV cell enhances the output electric power produced by the PV cell.